Apparatus for separating or breaking a pile of sheets, such as piles of cardboard or paper sheets, into stacks of sheets are known in the art.
There are at least three main types of apparatus for separating a pile of sheets into stacks of sheets according to a tear surface, which tear surface is constituted by tear paths, usually coplanar tear lines, each sheet in the pile of sheets being provided with at least one tear path along its surface. The tear path may be e.g. a pre-cut line, comprising nicks alternating with notches along the line.
The first type of apparatus comprises at least a first clamp for clamping the pile at a first side of the tear surface and a second clamp for clamping the pile at a second side of the tear surface. Once the clamps are closed, i.e. the pile is brought in a fixed position relative to the clamps, the clamps are moved sidewise, i.e. in a direction substantially perpendicular to the tear surface. This causes the tear lines to break by e.g. causing rupture of the nicks of the pre-cut lines. A first stack of sheets is clamped by the first clamp, and the remainder of the pile of sheets forming the second stack of sheets, is clamped by the second clamp. The maximum height of the stacks that can be broken is dependent on the quality and strength of the sheets to be broken. If sheets with a high breaking strength in a direction parallel to the sheet surface are to be broken, excessive forces are needed to move the clamps sidewise. Often the height of the stack is to be kept small in order to keep the force needed within the machine capabilities. If sheets, even with a low breaking strength in a direction parallel to the sheet surface, but with a low compression resistance in a direction perpendicular to the sheet surface are to be broken, the clamping force applied to the tack is to be kept limited in order to avoid the outer sheets to be damaged by the compression applied by means of the clamps.
Further, in order to prevent slippage of sheets one to the other, and slippage between sheets and surfaces of the clamps contacting the surface, a significantly large clamping force may be required to achieve breaking without sheets displacing one relative to the other. These high clamping forces applied to the pile of sheets can damage the sheets positioned near the upper and lower side of the pile, i.e. near the clamps. As an example, imprints of the clamping surfaces in the surfaces of the outer sheets may occur.
The second and third type of apparatus is apparatus comprising at least a first clamp for clamping the pile at a first side of the tear surface and a second clamp for clamping the pile at a second side of the tear surface. Once the clamps are closed, the clamps are rotated one relative to the other about an axis, which axis is usually substantially parallel to, and usually coplanar with the plurality of tear lines. This causes the tear lines to break, thereby providing a first stack of sheets clamped by the first clamp, and the remainder of the pile of sheets forming the second stack of sheets, clamped by the second clamp. The first of the clamps has an upper element with a contact surface for contacting the upper surface of the stack of sheets. The first of the clamps further has a lower element with a sustaining surface for sustaining the lower surface of the stack.
The second type of apparatus has the axis of rotation positioned beneath the sustaining surface of the first clamp. The clamps rotate one relative to the other about an axis, whereby the distance between the contact surface of the upper element and the second clamp is increased to a larger extent than the increase of the distance between sustaining surface of the lower element and second clamp. This causes the tear lines to break gradually through the height of the pile, starting with the breaking of the tear line closest to the upper element of the first clamp. An example of such apparatus of the second type is shown in U.S. Pat. No. 6,019,267 and EP1541304A1.
For apparatus according to the second type, the sustaining area of the second element of the first clamp is to be dimensioned sufficiently large, enabling holding of stacks of sheets with the largest surface area for which the apparatus is designed. Especially in the longitudinal or machine direction of the apparatus (this is the direction according to which the pile of sheets move through the apparatus), the longitudinal dimension of the second element is to be sufficiently large to be able to hold stacks of sheets with the largest length in longitudinal direction for which the apparatus is designed.
In case the contact surface of the upper element of the first clamp has a smaller length in the longitudinal direction than the lower sustaining element, no unclamped stack of sheets may be present on the sustaining surface during breaking of a stack of sheets. Stacks, which are not clamped during moving of the clamps, may become misaligned by mutually displacing sheets within this unclamped stack. Hence, when stacks of sheets with a relatively short length in longitudinal direction are to be produced, each stack of sheets is to pass along a relatively long longitudinal path to remove the stack from the sustaining surface, before a next stack may be broken off. The time taken up by moving stacks along the long surface area of the second element, causes the yield or stacks per time unit of the apparatus to be low when producing stacks with small length in longitudinal direction.
An alternative is an apparatus with increased length in the longitudinal direction of the contact surface of the upper element, in order to clamp also one or more stacks already broken off. This however causes larger machine elements, e.g. larger clamps, to be moved, hence may cause the energy consumption of the apparatus to increase.
The third type of apparatus has the axis of rotation positioned above the clamps. The first of the clamps has an upper element with a contact surface contacting the upper surface of the stack of sheets. The first of the clamps further has a lower element with a sustaining surface for sustaining the lower surface of the stack. The clamps rotate one relative to the other about an axis, whereby the distance from the sustaining surface of the lower element of the first clamp relative to the second clamp is increased to a larger extent than the distance between contact surface of the upper element and second clamp. This causes the tear lines to break gradually through the pile, starting with the tear line closest to the lower element of the first clamp. An example of such apparatus of the third type is shown in U.S. Pat. No. 6,776,748 B2, more particular in FIG. 1 and FIG. 6 of U.S. Pat. No. 6,776,748 B2. Once the clamping members are brought down to clamp the pile of sheets, the left-hand part of the frame of the apparatus of U.S. Pat. No. 6,776,748 B2 is rotated about the axis in order to break the tear lines present in the sheets of the pile of sheets in transverse direction.
When thin piles are broken using apparatus of the third type, it is noticed that the displacement of the lower clamping elements, which sustain the pile of sheets and the stack of sheets broken off, becomes similar to the movement of the clamps in apparatus according to the first type. The forces applied to the thin piles become similar to the forces used by apparatus according to the first type. Hence the forces applied to thin piles may also tend to damage the surfaces of the sheets.
Further, as the angle of rotation is usually set for higher piles, the displacement of the clamps is usually too large when thin piles are to be divided in to stacks of sheet. Operation time is lost during unnecessary displacement of the clamps one relative to the other. This causes time losses between consecutive breaking operations of the apparatus. Hence the operation time of the apparatus is not optimally used.